International Standards Organization (ISO) Standard 11785, “Radio frequency identification of animals—Technical Concept” (1996) relates to radio frequency identification (RFID) systems that comprise low frequency reader devices and passive, inductively powered identification transponders or tags (“ID tags”). ISO Standard 11785, the disclosure of which is incorporated herein by reference, defines the technical principles for communications between reader devices and two types of electronic passive identification transponders referred to as “HDX” and “FDX-B”. These transponders contain identification information stored in binary form, which is conveyed to the reader when the transponder is suitably activated by the reader. In RFID systems of this type, the reader device generates a tag activation signal and receives identification data signals from the ID tag using a resonant frequency antenna circuit. The reader device can use a single antenna coil to perform these functions or can use separate transmit and receive antenna elements. Readers in which a single antenna performs both transmit and receive functions are very economical and efficient, and comprise the most common design implementation of low-frequency RFID readers. However, when a single antenna is designed to serve these multiple purposes, the antenna's design characteristics are inevitably compromised, and the antenna performs less efficiently than when it can be designed optimally for a single purpose.
Passive low frequency RFID readers and tags are based on operating principles that are well-known to those of ordinary skill in the art. These operating principles are described in, for example, U.S. Pat. No. 3,299,424 (Vinding—1967), U.S. Pat. No. 3,713,146 (Cardullo—1973), and U.S. Pat. No. 5,053,774 (Schuermann—1991), and in textbooks such as “RFID Handbook” (Finkenzeller—1999).
Because the RFID reader's activation signal is a single frequency and because the identification tag signal comprises multiple frequencies, it is advantageous to adjust the resonant frequency tuning of the reader's antenna circuit at the appropriate times so that the antenna is always optimized for either transmitting the activation signal frequency or for receiving the transponder identification signal frequencies.
U.S. Pat. Nos. 5,168,282 and 5,289,199 (Viereck—1992 and 1994) disclose a parallel-coupled antenna resonant circuit that dynamically switches between two resonant frequencies as a function of the voltage in the circuit. An antenna coil is coupled in parallel to a resistor and a first capacitor. A second parallel capacitor is switched in and out of the circuit to yield a higher resonant frequency when receiving and a lower resonant frequency when transmitting.